1. Field of the Invention
The present invention relates to a semiconductor memory device and a method for fabricating the same, and more particularly, to a semiconductor memory device having a storage node contact plug and a method for fabricating the same.
2. Description of the Related Art
In semiconductor memory devices such as DRAM (Dynamic Random Access Memory) devices, a variety of contact holes are employed, including for example, a pad contact hole, a bit line contact hole, a storage node contact hole, a metal contact hole and a via contact hole. Among these contact holes, the storage node contact hole is very small, because it must be formed in a narrow region between the bit lines, and difficult to form, because it is created by deeply etching an interlayer dielectric layer. Moreover, to form the storage node contact hole in the narrow region between the bit lines, high level and precise alignment skills are required in the photolithography process. It is very difficult to reproducibly form a storage node contact hole since an alignment margin of 30 nm or less is required for a design rule of 0.15 μm or less.
Accordingly, a method for forming a storage node contact hole using a self-align contact etching process has been proposed. In the self-align contact etching process, after covering a bit line with a silicon nitride layer, the storage node contact hole is formed by etching an interlayer dielectric layer to be aligned at the silicon nitride layer, by taking advantage of the etching selectivity of the interlayer dielectric layer with respect to the silicon nitride layer. Next, a storage node contact plug material layer is formed in the storage node contact hole and then the storage node contact plug material layer is etched back, thereby forming a storage node contact plug. Thereafter, a storage node of a capacitor is formed on the storage node contact plug.
In highly integrated semiconductor memory devices, since the storage node is formed of a metal layer, the storage node plug material layer is also formed of a metal layer, such as a tungsten layer or a titanium nitride layer, in the self-align contact etching process.
However, when tungsten is used as the storage node contact plug material, the etching selection ratio (etching selectivity) of the storage node contact plug material layer (i.e., tungsten) with respect to the silicon nitride layer covering the bit line is poor. As a result, the storage node contact plug material layer made of tungsten is damaged, rather than being selectively etched. Such damage may cause a short circuit between the storage node contact plug and the bit line. Moreover, as the design rule decreases, the silicon nitride layer becomes thinner. Thus, if the silicon nitride layer is completely destroyed, margins for the self-align contact etching process are significantly decreased.
If the storage node contact plug material layer is formed of a titanium nitride layer, cracks may occur due to a large amount of stress at the titanium nitride layer when depositing the titanium nitride layer to have a thickness greater than a certain value. Moreover, the cracks may propagate to the interlayer dielectric layer, thereby causing serious problems.